Fig. 4: The relationships between groundwater nitrate concentrations and influencing factors, including groundwater aquifer depth, geographical factors (including ecoregion, latitude, and longitude), and climatic factors (including MAP and MAT).
a A decision tree-heatmap for predicting the threshold for nitrate concentration based on environmental factors associated with anthropogenic activities. b, c Structural equation models describing the direct, indirect, and total effects of aquifer depth, geographical factors, climate factors, and hydrochemical characteristics of nitrate. Path coefficients (i.e., regression coefficient) and coefficients of determination (R2 = 0.63) represent the effect size (λ) of the relationship (numbers adjacent to the paths) and the proportion of variance explained by the relationships in the model. The direct effect represents the impact of the independent variable on dependent variable, while indirect effect reflects its influence through one or more intermediate variables. The total effect is the combined effect of independent variable on dependent variable, which is the arithmetic sum of direct and indirect effects. The thickness of the arrows reflects the relative magnitudes of the standardized path coefficients. Red, blue, and gray color denotes positive, negative, and irrelevant effects, respectively. The dashed lines represent insignificant relationships; *P <  0.05, **P <  0.01, and ***P <  0.001.
